Comparison of Radiation and Convection-Based Calibration of Fast-Response Heat Flux Sensors

In order to optimize heat transfer in turbomachinery, combustion engines, high-speed flow applications etc., there is a strong demand for highly time-resolved heat flux measurements at the surface. Typically, wall-mounted heat flux gauges based on different measurement principles (thin-films, thermocouples, differential layer devices, etc.) are used, because optical methods do not reach sufficient levels of accuracy or spectral resolution for comparisons to numerical simulations, especially for complex geometries and regions close to the wall.

The atomic layer thermophile (ALTP) is a representative of a wall-mounted gauge with very high temporal resolution. The sensor exhibits time constants in the microsecond range and was used e.g. in studies of the heat transfer augmentation of internal passages of turbine blades [1], as well as in super- and hypersonic transition studies in several facilities [2].

A major challenge for all heat transfer gauges is the determination of its sensitivity with sufficient accuracy. Typical accuracies of sensors are between ±10 to 20 % [1, 2]. Normally, the calibration is carried out only for one single heat transfer mechanism - most commonly a radiation-based procedure. The validity of the calibrated sensitivity for another heat-transfer mechanism is not evident and studies that compare sensitivities for different types of heat-transfer are very sparse. In this paper, the calibration of different heat flux gauges in two different environments, a radiation-based setup and one mainly convection-driven, are investigated and compared. The ALTP (based on the transverse Seebeck effect) is compared with a differential layer device (HFM-8E, Vatell), a coaxial thermocouple, a thin-film gauge (both Müller Instruments) and a slug-calorimeter. To the authors’ knowledge, this is the first direct comparison of heat flux gauges with fundamentally different working principles calibrated with both, a convection- and a radiation-based procedure. It should be noted, that a dynamic calibration procedure of the ALTP sensor, allowing the correction over the entire spectrum was presented earlier [2].

The radiation-based calibration procedure intends to relate the absorbed power of a laser beam with a measured heat flux density. The setup consists of a CO₂-laser with a microlens array for beam profile homogenization and an attenuator for the variation of the laser power. With an accurate calibrated power meter device placed behind an adaptable aperture, the reference heat flux density can be exactly applied to the sensors specific surface. The accuracy of this technique is going to be discussed.

The convective calibration setup is based on a hot air jet (~700 °C) impinging on a flat plate. The jet is contracted in a nozzle for homogenization and the heat flux sensors are placed in the stagnation region of the baffle plate. Measurements showed the homogeneity of the heat flux in the stagnation region.  The heat flux itself is of mainly convective origin. By modifying the distance between nozzle exit and baffle plate, the heat flux density in the stagnation region is varied. Performing a substitution calibration technique with different travel distances enables a broad measurement spectrum over one order of magnitude for direct comparison of the different heat flux gauges.

Both setups are capable of delivering heat flux densities up to 8 W/cm² with low repetition errors in the range of ±6 %. The experimental results show a comparison of wall-mounted heat flux sensor based on fundamentally different working principles. In addition, the sensitivity of the gauges are compared for a purely radiative and mainly convective environment.

[1] Roediger, T.; Jenkins, S.; Knauss, H.; v. Wolfersdorf, J.; Gaisbauer, U.; Kraemer, E. : Time-Resolved Heat Transfer Measurements on the Tip Wall of a Ribbed Channel Using a Novel Heat Flux Sensor - Part I & II, ASME Journal of Turbomachinery, 130, 1, 2008.

[2] Roediger, T.; Knauss, H.;., Estorf, M.; Schneider, S.P., Smorodsky, B.V. : Hypersonic instability waves measured using fast-response heat-flux gauges, AIAA Journal of Spacecraft & Rockets, 46, 2, 2009.